1. Field of the Invention
The present invention relates generally to a thermionic cathode, and is directed more particularly to a thermionic cathode suitable for use with an electronic tube such as a color cathode ray tube and so on.
2. Description of the Prior Art
In the prior art, an indirectly heated thermionic cathode has usually been employed as the cathode of a color cathode ray tube. For example, as shown in FIG. 1, the cathode used in a color cathode ray tube of the Trinitron (Trade Mark) type or three-beam single gun type is formed of three cathode sleeves 3R, 3G and 3B each of which has a heater 2 therein and which are aligned in the horizontal direction within a cup-shaped first grid G.sub.1 common to the three cathode sleeves 3R, 3G and 3B. The cathode sleeves 3R, 3G and 3B are covered, at their ends facing an end plate 5 of the grid G.sub.1, with caps 4 which will serve as the base metal of the cathode, and a cathode material 4.sub.1 is coated on the top surface of each of the caps 4 to form a respective thermionic emission face. The cathode sleeves 3R, 3G and 3B pass through apertures or bores 7R, 7G and 7B formed in a ceramic base plate 6, which is inserted in the grid G.sub.1 and their thermionic emission faces are positioned to oppose three apertures 8R, 8G and 8B, respectively formed through the end plate 5 of the grid G.sub.1. The cathode sleeves 3R, 3G and 3B are fixedly supported by supporting pins (not shown) planted on the base plate 6 through supporting tabs (not shown), respectively, and the heaters 2 are supported in such a manner that the ends of each heater 2 are welded to the corresponding pair of heater rests (not shown) formed on the base plate 6. The base plate 6, which supports the cathode sleeves 3R, 3G and 3B and the respective heaters 2, is disposed in the grid G.sub.1 and spaced from the end plate 5 of the grid G.sub.1 through a spacer 9 and is fixed in the grid G.sub.1 by a retainer 10.
Further, in order to avoid the leakage into each other of thermions emitted from the respective cathodes, which leakage would cause crosstalk, a pair of shield plates or a cylindrical shield plate 1 are attached to the inner surface of the end plate 5 to isolate the thermion emitting portions of the respective cathode sleeves 3R, 3G and 3B.
As described above, the prior art indirectly heated type thermionic cathode is formed of a number of parts so that it is trouble-some to assemble the parts. Especially, in the case of a color cathode ray tube in which a plurality of electron beams are necessary or a plurality of cathodes are used, its productivity is much lowered thereby.
In order to avoid the defects of the above prior art cathode, a cathode formed of a laminated structure has been proposed for manufacture by a so-called thick-film print-circuit technique in which a heater and a cathode are successively coated on a base plate. However, the prior art cathode laminated structure is lacking in reliability.